#include <stdio.h>

int main(int argc, char *argv[]) {

    int n = 0;
    scanf("%d", &n);
    int sum = 0;
    for (size_t i = 1; i <= n; i++)
    {
        if(i%2!=0)
            sum += i;
    }
    printf("%d", sum);

    return 0;
}